The invention concerns the area of processing a sequence of images, and more particularly the processing of a sequence of medical images such as MRI images.
Purely visual, qualitative analysis of medical images is difficult and requires much experience on the part of the practitioner. Current possibilities of processing digital data provided by medical equipment, such as imaging equipment, makes it possible to use a method to assist the practitioner, notably to evaluate the volume of components which the practitioner wishes to examine, these generally being anomalies (lesions, tumour, stroke event, etc.). Algorithms of greater or lesser performance to adjust images, segment and automatically quantify anomalies have been developed. The objective, in particular, is to quantify the progress of a disease reliably, objectively and in reproducible manner, and to reduce the time spent by the practitioner in analyzing images.
The analysis of medical images has become an important point in medical diagnosis and in the decisional aspect of any surgical procedure. Image analysis has been substantially improved, in particular through the possibility to make digital interpretation of medical images (such as MRI images). A digital image is in fact a set of data having a certain position and a certain value (e.g. grey scale). Digital processing of images amounts simply to handling these data with a view to obtaining desired information.
In general, image-assisted medical diagnosis makes it possible to generate a series of successive images representing a set of successive slices of a body organ or body part of a patient. Since analysis is essentially based on digital image data, the whole problem is that of knowing how to interpret reality from these data, bearing in mind that such data is only a more or less true representation of reality. In addition, the difficulty in analyzing a series of images lies in the ability to relate between the elements identified in a given image and those identified in all the other images of the series.
At the present time, the calculation of volumes in medical images of MRI type for example is not calculated directly in 3D. It is carried out with the aid of two-dimensional computer tools (measurement of length with a computer screen ruler, measurement of an ellipse, a polygon, a disc) in each image section (axial, sagittal, and coronal), after which the practitioner manually carries out an approximate calculation. Since the volume of anomalies (stroke event or tumour for example) is rarely of simple geometric shape, this extrapolation method generates lack of precision in the 3 dimensions and therefore leads to an amplified delta error.
In particular by denoting: n                A the surface to be measured in the axial section and Δa its measurement uncertainty in this sectional plane;        S the surface to be measured in the sagittal section and Δs its measurement uncertainty in this sectional plane;        F the surface to be measured in the coronal section and Δf its measurement uncertainty in this sectional plane,the calculation of volume V in medical images is made using the following formula:V=A*S*F with an uncertainty of Δv=V*(Δa/A+Δs/S+Δf/F)        
One objective of the invention is to propose an improved method and device to process a sequence of images compared with the state of the art, with which it is possible in particular to improve the accuracy and reliability of volume calculation using said images. For this purpose, the invention proposes a method to transmit a label between two images, the method comprising the following successive steps:                providing a first image, the first image comprising several sets of connected points, each set being characterized by a label,        providing a second image,        determining from the second image several sets of connected points,        superimposing the two images to determine common parts and non-common parts between the sets of the first and second image,        to each common part of the second image, giving the label of the set in the first image with which said part is common,        to each non-common part of the second image in contact with a single set of connected points in the first image, giving the label of said set,        to each non-common part of the second image not in contact with any set in the first image, giving a new label.        
Advantageously, but optionally, the invention comprises at least one of the following characteristics:                the method comprises an additional step in which each non-common part of the second image in contact with at least two sets of the first image having a different label, is deleted,        the method comprises an additional step in which a new label is given to each non-common part of the second image in contact with at least two sets of connected points in the first image having a different label,        the method comprises an additional step in which each non-common part of the second image in contact with at least two sets of the first image having a different label, is given the label of one of the two sets,        each label is characterized by a value, the method further comprising the following step:                    creating an equivalence between the labels of two sets of the first image in contact with a same set in the second image.                        several labels are incompatible so that no equivalence is created between these labels,        the step to determine the sets comprises at least one of the following processing steps: segmentation, application of a variance filter, region filling step.        
The invention also concerns a method to determine volumes using different images, each image representing successive, substantially parallel planes, the method comprising the following steps:                choosing an initial image among the group of images,        selecting points in the initial image whose volume it is desired to determine,        determining several sets of connected points in the initial image so that the selected points belong to the same set, said set having a primary label, the other sets having a secondary label,        implementing the label transmission method according to the invention, on all the images,        determining the volume corresponding to the sets of each image comprising said specific label.Advantageously, but optionally, this method comprises at least one of the following characteristics:        the volume determination step comprises a step to integrate the surface areas of the sets of each image comprising said specific label.        
The invention also concerns a method to determine volume from medical CT-scan or MRI images, comprising all the steps of the method to determine volumes according to the invention. The invention also concerns a method to determine volume from a CT-scan or MRI image of a patient's brain, the method comprising all the steps of the method to determine volumes according to the invention, characterized in that it previously comprises the following steps:                in the initial image, determining the set corresponding to the patient's cranium,        in the initial image, determining the set corresponding to the patient's brain.        
The invention also concerns a device to determine volume from medical images, for example CT-scan or MRI images, characterized in that it comprises means to implement all the steps of the method according to the invention. The invention also concerns a device according to the preceding claim, which further comprises an apparatus to acquire said medical images, such as MRI or scanning equipment. The invention also concerns a computer programme product comprising programme code instructions recorded on a medium which can be used in a computer, to implement all the steps of the method according to the invention.
Said method can advantageously be used for semi-automatic measurement (for example with a click by the practitioner on a region of interest and optionally colour enhanced by corresponding software), instant measurement, and to specify volumes from medical images. Said method can advantageously be dedicated to images of a brain MRI, applied in particular to measure tumours and stroke events. The method may also be dedicated to tumours of the pelvis. With said method it is possible, unexpectedly, to obtain fast, accurate measurements of the volume of an anomaly (stroke event, lesion, etc.) and can be used for more accurate decision on the treatment to be given; in particular to take a safe decision for a complex and/or hazardous operation. Said decision generally depends on a threshold volume. For example, in the case of a stroke, surgical procedure is decided if the volume of the anomaly exceeds one third of the brain MCA territory. Therefore, with a volume calculation method according to the invention, this trigger threshold for surgical procedure (such as thrombolysis) is reliable and reproducible.